The correct answer is A) slavery.
John Brown was an avid abolitionist who made it his life goal to get rid of the institution of slavery. Brown was a firm believer that the only way to get rid of slavery in the US was through bloodshed. This is why he organized an invasion of a federal arsenal (aka Harper's Ferry). The goal of this invasion was to obtain guns and ammunition to start a nation wide slave rebellion that would result in the destruction of slavery within the US. Ultimately, this raid was unsuccessful and resulted in John Brown being sentenced to death.